I had the blood tests run and it showed I was deficient. So the doctor put me on 5000iu, and immediately I showed symptoms of overdose. Severe headache, increased heart rate, vomitting, difficulty breathing, pain in the kidneys, severe joint pain (above and beyond what I normally experience), and a few seizures. All within three days of taking it. Another blood test revealed my numbers had shot through the roof even though they were considered quite low to begin with, just don't remember the exact numbers. A few months after I stopped taking it she ran another blood test just to make sure there wasn't some kind of mix up with the first test and my numbers were right back down again matching the first test...

She didn't put me back on the supplements though, only suggest I eat a bit more broccoli or add another glass of milk to my daily routine. Aside from water soluble vitamins it was strongly suggested I stay away from any supplements. Especially since I've had severe reactions to food fortified with iron from the hemochromatosis, but also severe reactions to taking vitamin C supplements. One dose from a halls vitamin C lozenge equals several cankersores in the mouth, tinny taste in the back of the throat, acid reflux, vomiting and confusion. And yet undoctored orange juice is fine. (And it's not just the halls, just that was the last one I tried, any vitamin C supplements had done that, I stay far away from them now).

But that's what a severe overdose can cause with vitamin D at any rate. Not something to mess with too much that's for sure, but then I'm sure for 99.9% of the population it will take sooo much more than 5000ui to cause such a severe reaction, my body's just messed up from the word go.